Question

The graph shows the distance y, in miles, of a moving train from a station over a certain period of time, x, in hours: A graph titled Distance Vs Time is shown with Time in hours labeled on x-axis and Distance from Station in miles labeled on y-axis. The scale on the x-axis shows the numbers 1, 2, 3, 4, 5, 6, 7, and the scale on the y-axis shows the numbers 0, 40, 80, 120, 160, 200, 240, 280, 320. The graph shows a straight line joining the ordered pairs 0, 80, and 1, 120, and 2, 160, and 3, 200, and 4, 240. What is the speed (in miles per hour) of the train and why?
 80 miles per hour, because speed is the initial value of the function 120 miles per hour, because speed is the distance traveled in unit time 240 miles per hour, because speed is the distance traveled in a certain time 40 miles per hour, because speed is the rate of change of distance

Answer 1

The speed of the train is equal to the slope. Why? Because speed = distance/time and because slope = rise/run. In this case, the "rise" is the change in distance while the "run" is the change in time. 

At x = 0 hours, the distance from the station is y = 80 miles. One hour later, at x = 1, the distance is now y = 120 miles. 

So,
rise = change in distance = 120-80 = 40 miles
run = change in time = 1-0 = 1 hour
slope = rise/run = 40/1 = 40

Therefore the speed of the train is 40 miles an hour

-------------------------------

Alternatively you can use the slope formula to get...

m = (y2-y1)/(x2-x1)
m = (120-80)/(1-0)
m = 40/1
m = 40

-------------------------------

Whichever method you use, the final answer is 40 mph